Transposon-induced mutants in Escherichia coli K-12 and Salmonella typhimurium LT2 are being utilized to investigate the organization and regulation of selected operons. Mutations generated by transposons are polar upon downstream expression, permitting gene order to be readily determined and internal promoters to be detected. We have found that the polycistronic isoleucine-valine (ilv) operon has four structural genes and two internal promoters: p1OGp2Ep3DA. The location and regulation of the genes coding for the isozymes of acetohydroxy acid synthase is being investigated. With one transposon, Mu d(Ap,lac) it is possible to assay the level of transcription of the gene into which this transposon is inserted. This property is being used to study the regulation of the thyA gene in E. coli.